In recent years, as digital technology has progressed, it has become increasingly common to process or store large amount of data at high speed. Accordingly, there is demand for highly integrated, high performance semiconductor device for use in electronic devices.
Accordingly, in order to realize high integration of semiconductor storage devices (DRAM), extensive research and development have been carried out for a technology using a ferroelectric material or a high dielectric constant material instead of a silicon oxide or a silicic acid nitride as a capacitor insulating film of a capacitor element constituting DRAM.
Furthermore, in order to realize a non-volatile RAM which can perform both write and read operation at low voltage and high speed, active research and development have been made for a ferroelectric memory (FeRAM) using a ferroelectric film having a spontaneous polarization characteristic as a capacitor insulating film.
Ferroelectric memory (FeRAM) stores information utilizing a hysteresis characteristic of the ferroelectric. In ferroelectric memory, ferroelectric capacitors having a ferroelectric film are provided for each memory cell as a capacitor dielectric film between a pair of electrodes. Polarization occurs according to applied voltage between electrodes in the ferroelectric material, and after the applied voltage is removed, a spontaneous polarization still remains. When the polarity of the applied voltage is reversed, the polarity of the spontaneous polarization also reverses. Accordingly, if spontaneous polarization is detected, information can be read out. Furthermore, when compared with flash memory, ferroelectric memory can operate under a low voltage, which makes it possible to write at high speed at a power-thrifty state.
The ferroelectric film used in FRAM is formed from a PZT series material such as lead zirconium titanate (PZT), La doped PZT (PLZT) or the like, a Bi-layer structured compound or the like such as SrBi2Ta2O9 (SBT, Y1), SrBi2(Ta, Nb)2O9 (SBTN, YZ) or the like.
Conventionally, a sol-gel method or a sputtering method has been used as a film formation method of a ferroelectric thin film. By these film formation methods, an amorphous phase ferroelectric film is formed above a bottom electrode film, and then, the ferroelectric film is crystallized into a Perovskite structured crystal by heat treatment. After the crystallization of the ferroelectric film, a top electrode film is formed. During this stage, the ferroelectric film is vulnerable to physical damage mainly by high energy sputtering particles at this time. As a result of such damage, a portion of the ferroelectric film crystal structure may be destroyed and the characteristics of the capacitor element are caused to deteriorate.
Therefore, in order to recover such deterioration of capacitor element characteristics, treatment like the following has been conventionally carried out. In a first conventional method, heat treatment is carried out in an oxygen atmosphere after patterning the top electrode film. In a second conventional method, heat treatment is carried out in an oxygen atmosphere after patterning the top electrode film and the ferroelectric film. In a third conventional method, heat treatment is carried out in an oxygen atmosphere after forming a capacitor element by patterning. In these treatments, oxygen recovers the crystallinity of the ferroelectric film.
Furthermore, in a conventional method, after formation of the capacitor element, an aluminum oxide film covering the capacitor element is formed as a diffusion preventing film to prevent the ferroelectric film from hydrogen deterioration. Consequently, in order to recover damage to the ferroelectric film sustained during patterning and during formation of the diffusion preventing film, heat treatment is again carried out in the oxygen atmosphere. Thereafter, an interlayer insulating film is formed.
Another method is disclosed in Patent Document 1 (Japanese Patent Application Laid-open No. Hei 5-251351). In the method disclosed in Patent Document 1, various types of heat treatment are carried out on the ferroelectric film in the oxygen atmosphere, aiming at obtaining a ferroelectric film having no oxygen holes, no generation of leakage current even by applying an electric field, and maintaining a dielectric constant. As the heat treatment, a method of irradiating ultraviolet rays to ozone or oxygen gas, heat treatment in an oxygen atmosphere under high pressure, and heat treatment using ozone gas can be cited.
Still another method is disclosed in Patent Document 2 (Japanese Patent Application Laid-open No. 2002-305289). In the method disclosed in Patent Document 2, by forming a PZT film which contains more Pb in compositional ratio than the PZT film as the capacitor insulating film after heat treatment to recover from damage to a ferroelectric capacitor, a side wall film is formed above a side surface of the ferroelectric capacitor. Due to such a side wall insulating film, the influence of hydrogen, H2O and the like existing in a TEOS film which is formed thereafter, is reduced so that deterioration of the PZT film characteristics is reduced.
However, even with the above-described methods, a ferroelectric capacitor having a sufficient characteristic cannot be obtained.
For instance, with the method disclosed in Patent Document 1, since crystal orientation of the obtained ferroelectric film is low, residual polarization is low.
With the method disclosed in Patent Document 2, when performing heat treatment to recover from damage to the ferroelectric capacitor, element having the highest vapor pressure moves out from an exposed portion of the ferroelectric film, and pin holes are formed in the ferroelectric film. In other words, when the PZT film is used, Pb loss comes out. Such Pb loss lowers a switching characteristic of the ferroelectric capacitor, and lowers an initial characteristic and retention. The occupation rate of the exposed portion where such loss occurs to the whole ferroelectric capacitor increases as the semiconductor integrated circuit becomes finer and finer. Accordingly, problems caused by such loss have been manifested recently. Furthermore, as the occupation rate increases, it becomes difficult to uniformly correct the Pb loss even when later forming a side wall film made from a PZT having plenty of Pb.
Moreover, with the method disclosed in Patent Document 2, due to the existence of the side wall film, a composition in the capacitor insulating film is easy to become uneven. Accordingly, the switching electric charge is apt to be lowered and imprint is apt to occur.
Patent Document 1
Japanese Patent Application Laid-open No. Hei 5-251351
Patent Document 2
Japanese Patent Application Laid-open No. 2002-305289